Currently, man has issues finding man pages for Nix-installed
application (also, `nix-env --help` doesn't work). The issue is caused
by custom `$MANPATH` set by my system. That makes man use it instead of
searching in default location.
Either of next lines workaround the issue:
```sh
unset MANPATH
export MANPATH=$HOME/.nix-profile/share/man:$MANPATH
```
This patch adds the later line to the `nix-profile.sh` if user has
`MANPATH` set. (Not clearing `MANPATH` as that would be disrespect of
user's preferences.)
As a side-effect, host's man might find man pages installed by Nix.
eef754813f